This invention relates to measurement of fuel cell impedance, and in particular, to a self-contained, portable apparatus for obtaining real-time measurements of a fuel cell""s internal resistance.
Fuel cells are becoming increasingly important as alternative energy sources as seen by the estimated 3 billion dollar market for fuel cells in 2000. This is due to their advantages over conventional power sources such as the battery and the internal-combustion engine. For instance, a fuel cell can supply electrical energy over a longer time period than a battery because it can be constantly supplied with air and fuel (i.e. hydrogen, reformed natural gas (hydrogen-rich gas) and methanol). Furthermore, a fuel cell does not run down or require recharging. Fuel cells are also high-efficiency devices, with efficiencies as high as 60 percent. This is much better than the internal-combustion engine which has an efficiency of up to 40 percent. Fuel cells also emit no noxious gases, since the fuel cell relies on a chemical reaction versus combustion, and generate very little noise when in operation. All of these features make the fuel cell highly desirable as power sources for automobiles, buses, municipal power generation stations, space missions and cellular phones.
To evaluate a fuel cell""s electrical efficiency, its internal resistance is determined which is achieved through AC Impedance measurement. This measurement is important because it allows for the examination of various physical and chemical characteristics of the fuel cell. This impedance measurement may also be used in a feedback mechanism to improve the fuel cell""s performance.
The literature indicates that complex impedance measurements on fuel cells can only be performed using expensive bench-top laboratory equipment, consisting of many sub-systems interfaced with one another. For example: T. E. Springer, T. A. Zawodzinski, M. S. Wilson and S. Gottesfield, xe2x80x9cCharacterization of polymer electrolyte fuel cells using AC Impedance spectroscopyxe2x80x9d, Journal of the Electrochemical Society of America, 143(2), p. 587-599, 1996; J. R. Selman and Y. P. Lin, xe2x80x9cApplication of AC impedance in fuel cell research and developmentxe2x80x9d, Electrochemica Acta, 38(14), p. 2063-2073, 1993; B. Elsener and H. Bolmi, xe2x80x9cComputer-assisted DC and AC techniques in electrochemical investigations of the active-passive transitionxe2x80x9d, Corrosion Science, 23(4), p. 341-352, 1983. Such known equipment is manually controlled, with no automation in place. No single known approach allows the use of a portable, integrated measurement system. In addition, no measurement equipment is integrated into these systems which permits modification of fuel cell operating parameters.
Furthermore, the patent literature shows that the measurement of complex impedance is primarily known for use on batteries. In addition, these patents only claimed to measure a single quantity, namely xe2x80x9cimpedancexe2x80x9d (U.S. Pat Nos. 4,697,134 and 5,773,978) or xe2x80x9cresistancexe2x80x9d (U.S. Pat Nos. 3,753,094, 3,676,770 and 5,047,722). The previous patent relating to measuring impedance on an electrochemical cell (U.S. Pat No. 6,002,238), not necessarily a fuel cell, used an entirely different, yet complicated approach. Furthermore, this approach could not be directly applied to fuel cells due to the high currents associated with the latter.
Thus the issues which still need to be addressed and improved in fuel cell impedance measurement are portability, fuel cell applicability, measurement variety and resolution, automation and cost.
Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel cell can generate power as long as the fuel and oxidant are supplied.
A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte and catalyst to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, typically air, oxygen enriched air or oxygen, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product such as water. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:
First Electrode: H2xe2x86x922H++2exe2x88x92
Second Electrode: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Fuel cells may be classified by the type of electrolyte, which is either liquid or solid. The present invention is primarily concerned with fuel cells using a solid electrolyte, such as a proton exchange membrane (PEM). The PEM has to be kept moist with water because the membranes that are currently available will not operate efficiently when dry. Consequently, the membrane requires constant humidification during the operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and air.
The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing basic units of a fluorinated carbon chain and sulphonic acid groups. There may be variations in the molecular configurations of this membrane. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified, but at the same time the membrane must not be over humidified or flooded as this degrades performances. Furthermore, the temperature of the fuel cell stack must be kept above freezing in order to prevent freezing of the stack.
Cooling, humidification and pressurization requirements increase the cost and complexity of the fuel cell, reducing its commercial appeal as an alternative energy supply in many applications. Accordingly, advances in fuel cell research are enabling fuel cells to operate without reactant conditioning, and under air-breathing, atmospheric conditions while maintaining usable power output.
Where a solid polymer proton exchange membrane (PEM) is employed, this is generally disposed between two electrodes formed of porous, electrically conductive material. The electrodes are generally impregnated or coated with a hydrophobic polymer such as polytetrafluoroethylene. A catalyst is provided at each membrane/electrode interface, to catalyze the desired electrochemical reaction, with a finely divided catalyst typically being employed. The membrane/electrode assembly is mounted between two electrically conductive plates, each which has at least one (fluid) flow passage formed therein. The fluid flow conductive fuel plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically connected in an electric circuit, to provide a path for conducting electrons between the electrodes. Electrical switching equipment and the like can be provided in the electric circuit as in any conventional electric circuit. The fuel commonly used for such fuel cells is hydrogen, or hydrogen rich reformate from other fuels (xe2x80x9creformatexe2x80x9d refers to a fuel derived by reforming a hydrocarbon fuel into a gaseous fuel comprising hydrogen and other gases). The oxidant on the cathode side can be provided from a variety of sources. For some applications, it is desirable to provide pure oxygen, in order to make a more compact fuel cell, reduce the size of flow passages, etc. However, it is common to provide air as the oxidant, as this is readily available and does not require any separate or bottled gas supply. Moreover, where space limitations are not an issue, e.g. stationary applications and the like, it is convenient to provide air at atmospheric pressure. In such cases, it is common to simply provide channels through the stack of fuel cells to allow for flow of air as the oxidant, thereby greatly simplifying the overall structure of the fuel cell assembly. Rather than having to provide a separate circuit for oxidant, the fuel cell stack can be arranged simply to provide a vent, and possibly some fan or the like to enhance air flow.
The present invention relates to a self-contained, portable apparatus used to measure the real and imaginary components of the complex impedance of a fuel cell at discrete frequencies. The system comprises a CPU, frequency synthesizer, a fuel cell, a load bank and measurement and acquisition circuitry. The CPU receives input parameters from a software program and sends the parameters to a signal generation device which produces an AC waveform with a DC offset that is used to remotely program a load bank. The load bank draws current from the fuel cell. The voltage across the fuel cell and the current through the fuel cell are measured by voltage and current sensing circuitry, then digitized and averaged by an oscilloscope or A/D converter. This recorded data is sent to the CPU where the AC phase lead or lag is calculated. Numerous outputs can then be displayed by the invention, including real impedance, imaginary impedance, phase differential, leading component, lagging component, current magnitude, voltage magnitude and applied AC voltage.
Another aspect of the invention is that this apparatus allows for real-time measurements which can be continuously updated. These measurements can be automated to improve the measurement speed and simplicity which makes it very useful in assessing a large number of fuel cells. In addition, the effects of parameter changes such as flow rates, fuel cell temperature, and humidification levels on fuel cell impedance may be easily monitored when a fuel cell is interfaced with this measurement system. This can permit the modification of fuel cell operating parameters.
In accordance with a first aspect of the present invention, there is provided an apparatus for determining at least the real component of a fuel cell""s complex impedance, the apparatus comprising, a load device, connectable to the fuel cell for drawing a current from the fuel cell; a voltage sensing circuit connectable across the fuel cell for in use sensing direct and alternating components of a voltage across the fuel cell; a current sensing circuit, for in use sensing direct and alternating components of a current drawn from the fuel cell by the load device; wherein the load device is configured to draw a direct current with a superimposed alternating current signal; and an analysis device, connected to the voltage sensing circuit and the current sensing circuit, for analyzing the detected direct and alternating components of the voltage and the detected direct and alternating components of the current to determine real and imaginary parts of the fuel cell impedance at different frequencies, whereby the real part of the fuel cell impedance can be determined.
In accordance with another aspect of the present invention, there is provided a method of determining at least a real component of a fuel cell""s complex impedance, the method comprising:
(i) applying a load to the fuel cell so as to draw a current from the fuel cell, the current comprising a direct current component and an alternating current component;
(ii) varying the frequency of the alternating current component;
(iii) measuring the direct and alternating components of the voltage across the fuel cell and the direct and alternating components of the current drawn from the fuel cell;
(iv) from the measured current and the measured voltage determining the real impedance and imaginary impedance at different frequencies; and
(v) from the real and imaginary impedances determined in the preceding step, determining the frequency at with the imaginary impedance is zero and determining the real impedance at said frequency, indicative of the total real impedance of the fuel cell.